Synthesis and Structures of Plutonyl Nitrate Complexes: Is Plutonium Heptavalent in PuO sub(3)(NO sub(3)) sub(2 ) super(-)?

Gas-phase plutonium nitrate anion complexes were produced by electrospray ionization (ESI) of a plutonium nitrate solution. The ESI mass spectrum included species with all four of the common oxidation states of plutonium: Pu(III), Pu(IV), Pu(V), and Pu(VI). Plutonium nitrate complexes were isolated in a quadrupole ion trap and subjected to collision-induced dissociation (CID). CID of complexes of the general formula PuO sub(x)(NO sub(3)) sub(y ) super(-) resulted in the elimination of NO sub(2) to produce PuO sub(x+1)(NO sub(3)) sub(y-1) super(-), which in most cases corresponds to an increase in the oxidation state of plutonium. Plutonyl species, Pu super(V)O sub(2)(NO sub(3)) sub(2) super(-) and Pu super(VI)O sub(2)(NO sub(3)) sub(3) super(-), were produced from Pu super(III)(NO sub(3)) sub(4) super(-) and Pu super(IV)(NO sub(3)) sub(5) super(-), respectively, by the elimination of two NO sub(2) molecules. CID of Pu super(VI)O sub(2)(NO sub(3)) sub(3) super(-) resulted in NO sub(2) elimination to yield PuO sub(3)(NO sub(3)) sub(2 ) super(-), in which the oxidation state of plutonium could be VII, a known oxidation state in condensed phase but not yet in the gas phase. Density functional theory confirmed the nature of Pu super(V)O sub(2)(NO sub(3)) sub(2) super(-) and Pu super(VI)O sub(2)(NO sub(3)) sub(3) super(-) as plutonyl(V/VI) cores coordinated by bidentate equatorial nitrate ligands. The computed structure of PuO sub(3)(NO sub(3)) sub(2 ) super(-) is essentially a plutonyl(VI) core, Pu super(VI)O sub(2) super(2+), coordinated in the equatorial plane by two nitrate ligands and one radical oxygen atom. The computations indicate that in the ground spinEMDASHorbit free state of PuO sub(3)(NO sub(3)) sub(2 ) super(-), the unpaired electron of the oxygen atom is antiferromagnetically coupled to the spin-triplet state of the plutonyl core. The results indicate that Pu(VII) is not a readily accessible oxidation state in the gas phase, despite that it is stable in solution and solids, but rather that a Pu(VI)O. bonding configuration is favored, in which an oxygen radical is involved.